The use of carbon fiber reinforced glass matrix composites (CFRGM composites) as replacements for metal has become common in services requiring high strength, stiffness, and light weight. Products in which such composites have been used range from sporting goods to jet engines.
CFRGM composites typically comprise a glass or glass-ceramic matrix in which carbon fibers are imbedded. The carbon reinforcing fibers may be either continuous or discontinuous depending on the desired application. Continuous fibers generally extend for the entire length of a composite article, while discontinuous fibers, which are significantly shorter than continuous fibers, tend to provide more localized matrix reinforcement. As a result, continuous fiber CFRGM composites are often used for load bearing structural applications, while discontinuous fiber CFRGM composites are more suitable for nonload or low load bearing nonstructural applications, especially those in which parts must be fabricated into complex shapes. Such composites are described in commonly assigned U.S. Pat. Nos. 4,314,852 to Brennan et al. and 4,324,843 to Brennan et al. Articles made from carbon fiber reinforced composites may be formed in several ways, including by hot pressing in a shaped die as taught in commonly assigned U.S. Pat. No. 4,314,852 to Brennan et al.; by transfer molding as taught in commonly assigned U.S. Pat. No. 4,428,763 to Layden; or by injection molding as taught in commonly assigned U.S. Pat. Nos. 4,464,192 to Layden et al. and 4,780,432 to Minford et al.
The interaction between the carbon fibers and matrix material is responsible for the superior properties displayed by CFRGM composites. The fibers contribute to the composite's strength and elastic modulus by absorbing loads transferred from the matrix through fiber-matrix interfacial bonds. The fibers improve the composite's toughness by inhibiting or blunting the formation of cracks in the matrix. In addition, carbon fibers exposed at the surface of the matrix impart their good lubricating properties to the composite.
Despite their superior physical properties, all CFRGM composites are susceptible to carbon fiber oxidation, particularly when exposed to elevated temperatures. The problem is exacerbated by the presence of matrix microcracks which form during fabrication as a result of a thermal expansion mismatch between the glass matrix and carbon fibers. Microcracking is especially extensive in discontinuously reinforced composites because of the complex stress states arising from the random three-dimensional arrangement of the fibers. Matrix microcracks provide channels which permit oxygen to penetrate into the matrix, providing the opportunity for carbon fibers in the interior of the matrix to oxidize when exposed to elevated temperatures. Carbon fiber oxidation can quickly destroy the composite's strength and lubricity, making carbon fiber composites unsuitable for certain applications or requiring frequent replacement of parts constructed from these composites.
Accordingly it would be desirable to have a discontinuously reinforced CFRGM composite which resists the effects of oxidation and maintains its strength and lubricity, particularly at high temperatures.